The size and the diameter of a spark plug used for an internal combustion engine have been required to be reduced for the purpose of improvement in the degree of freedom in designing of the internal combustion engine and the like. For example, by reducing the diameter of the spark plug, the diameter of a mounting hole into which the spark plug is to be mounted can be reduced, and thus the degree of freedom in designing of an intake port and an exhaust port can be improved. By reducing the size and the diameter of the spark plug, the diameters of an insulator and a metal shell can be reduced, and thus the mechanical strengths of the insulator and the metal shell are reduced. A technique has been proposed in which a seal member is provided in order to improve sealability between an insulator and a metal shell in such a case, the seal member being provided between: a diameter reduction portion (specifically, a portion having an inner diameter that reduces toward a front side), of the metal shell, which is formed by a projecting portion projecting radially inward; and a diameter reduction portion, of the insulator, which has an outer diameter that reduces toward the front side. Specifically, the tilt of the diameter reduction portion of the metal shell with respect to an axial line of the spark plug is made gentler than the tilt of the diameter reduction portion of the insulator with respect to the axial line, so that load that is received from the seal member by the diameter reduction portion of the metal shell becomes smaller on the inner peripheral side than on the outer peripheral side. As a result, deformation of the projecting portion of the metal shell is suppressed. Prior art is disclosed in International Publication WO 2014/013654 (“Patent Document 1”).
Incidentally, when a spark plug is manufactured, force is applied to a part (e.g., rear end portion) of a metal shell so as to bend the part in order to fix an insulator to the metal shell. For example, the rear end portion of the metal shell is crimped. Such force is transmitted from the metal shell to the insulator so that the insulator can be pressed frontward against the metal shell. Accordingly, a diameter reduction portion of the insulator can press a diameter reduction portion of the metal shell frontward. The metal shell is sometimes deformed owing to such force. When a screw portion formed on the outer peripheral surface of the metal shell is deformed, it sometimes becomes difficult to appropriately mount the spark plug into a mounting hole of an internal combustion engine.